When a torsion box shelf fails, it is most often because the screws that hold the ledger onto the wall have bent. To avoid this, it's best to use stout (V8-in.-dia. or !/2-in.-dia) lag screws and washers, and to make certain that at least two-thirds of the screw shank will be anchored in the wall framing. You can countcrbore the screws or cut pockets for them to get deeper purchase for the threads. (See left photo, above.)
If you anticipate removing a T-box shelf at some point, you can attach it to the ledger by driving screws through the top and bottom skins. But if you want maximum strength, you will have to glue the ledger into its pocket. Use long clamping cauls on the top and bottom edges of the ledger, as shown in the right photo, above.
A few layers of veneer under the center of each caul will help to apply clamping pressure there, as you tighten the clamps at each end. Clamp a temporary vertical strut to the front edge of your shelf, as shown in the above right photo, to keep the shelf level while the glue sets.
When the time comes to remove the clamps, there's a standard method for
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